As it is well known, during the "turn-off" transient of an inductive load, overvoltages are developed which may result in irreversibly damaging the load-driving electronic components.
To obviate such a serious drawback, the prior art has proposed driver circuits for inductive loads which incorporate protection arrangements against overvoltages.
A typical example of a circuit of this kind is illustrated by FIG. 1, where an inductive load represented by an inductance L is shown driven by a power transistor T between a power supply line at a voltage Val and ground.
When the power transistor is driven to conduct, as by means of an appropriate base voltage Vc(t), a voltage drop V.sub.L appears across the inductance L which is equal to the difference between the supply voltage Val and the collector-to-emitter voltage Vce of the transistor operating in the saturation phase. As a result, a current I.sub.L flows through the inductance, which is represented by the following relation: EQU I.sub.L =(Val-Vce)/L.
Further, the amount of energy stored in the inductance will be: EQU (1/2)L I.sub.L.sup.2
Upon turning off, due to the natural inertia of the inductive load, the current I.sub.L tends to retain its value, and since under such conditions the transistor is in an open circuit state, the voltage at the collector rises to very high values, well above the supply voltage Val.
Accordingly, the transistor would be excited by an overvoltage which may cause it to fail.
To avoid such likely damage, a loop-back diode D has been proposed which is connected in parallel with the inductance. Upon turning off, the energy stored in the inductance is discharged as a current flowing through the diode D.
While being advantageous from certain aspects, this prior approach can only be adopted where switching speed is an unimportant limitation. The discharge time Ts offered by the prior approach actually is fairly long and often unacceptable for many practical applications, such as those for wire printers whose high printing rate requires very short current decay times.
To overcome this limitation, the prior art has proposed a second approach illustrated by FIG. 2 of the accompanying drawings.
A Zener diode Z is added and connected between the diode D and the power supply Val.
Upon turning oft; the voltage across the inductance L is VF and VZ added together, the two last-mentioned terms representing the voltage drop across the diode D and the Zener diode Z.
The decay time, or time Ts to discharge the current I.sub.L, is, therefore, reduced in accordance with the following relation: EQU Ts=L I.sub.L /(VF+VZ)
By adjusting the threshold value VZ, the value of the discharge time Ts can be set for a given inductance L and current I.sub.L. However, not even the latter prior approach is entirely devoid of disadvantages. For example, all of the energy stored in the inductance during the conduction period is lost to the Zener diode.
Dependent on the number and the characteristics of the inductive loads, the power lost to the Zener may be on the order of some tens of watts.